An Oceanic Indication That Earth's Climate Might Regulate Itself

By WILLIAM K. STEVENS

Published: May 7, 1991

The question fascinates scientists, especially at a time of concern and contention about global warming, and now climatologists have produced evidence that clouds act as a natural thermostat that keeps the temperature of the oceans' surface from rising above a certain point.

While the evidence comes primarily from the tropical Pacific Ocean, climate experts say it suggests that the earth's closely coupled ocean-atmosphere system could act as a more general thermostat to prevent any warming of the global climate from spiraling out of control.

The evidence raises the tantalizing possibility that a natural regulatory mechanism could set a ceiling on the global warming that scientists say will result from emissions of heat-trapping gases like carbon dioxide. At present rates of emission, they calculate, atmospheric concentrations of these greenhouse gases will double by the middle of the 21st century, eventually raising the average surface temperature of the earth by 2 to 9 degrees Fahrenheit. They say that a warming in the upper end of that range would have catastrophic results for the world's climate.

It is not yet known whether the effects of the predicted warming would be mitigated by the thermostatic effect that scientists say they have now demonstrated. But the new findings may rule out the direst scenarios about the greenhouse effect, in which global warming would escalate until it ultimately makes the earth inhospitable to life. Scientists and environmentalists who fear this possibility point out that this is precisely what happened on Venus. Ships and Satellites

"I cannot see how the planet can have a runaway greenhouse effect" given the new findings, said Dr. Veerabhadran Ramanathan, a climatologist at the Scripps Institution of Oceanography at La Jolla, Calif. With a colleague, Dr. William Collins, he presents the evidence for the thermostatic mechanism in the current issue of the British journal Nature.

Even so, said Dr. Ramanathan, global warming could still cause vast climatic disruptions. This, he said, is because the thermostatic mechanism, if it operated globally, would probably lead to a radical change in the large-scale circulation of huge rivers of air, like the jet streams, that create and define regional climates. These climates could be transformed in ways yet unfathomed. Moreover, he said, the temperatures of most of the world's oceans, unlike those in the tropics, are so far below the limit imposed by the thermostat that most of the earth's surface could warm signficantly before the thermostat kicked in globally.

The new evidence for a climatic thermostat comes from an analysis of data on changes in temperature and sunlight gathered by satellites and ships. Dr. Ramanathan and Dr. Collins studied the behavior of the ocean-atmosphere system over the eastern and central Pacific at the equator in 1987 when El Nino occurred. In this periodic event, naturally occurring changes in ocean currents caused the equatorial sea surface to warm by five to seven degrees. For Dr. Ramanathan and Dr. Collins, it provided a natural experiment in which to look at the relationship between temperature and clouds as the climate actually changes.

They found that as the sea surface grew warmer, water vapor increased substantially in the air. Water vapor is the most powerful greenhouse gas -- it amplifies the effects of other causes of heating, including both carbon dioxide and sunlight -- and in this case it produced what Dr. Ramanathan calls a "super greenhouse effect."

At the same time, huge, tall thunderclouds formed. As they reached freezing altitudes, the tops of the clouds turned into monstrous, flat "anvils" of cirrus clouds made of ice crystals. Together, at their maximum extent, the cirrus clouds covered nearly four million square miles of the earth's surface. Normally, cirrus clouds help trap heat in the atmosphere. But the thicker they become, the more sunlight they reflect. Eventually, said Dr. Ramanathan, "the cirrus becomes so thick and so highly reflective that it shuts off the sunlight reaching the ocean."

This was more than enough to counterbalance the warming below, the scientists say, and the net effect was to prevent the sea surface from warming further. Once the cooling begins, they found, the tall thunderclouds and the cirrus dissipate, and the process starts again. 'A Very Simple Effect'

The ceiling beyond which the ocean warms no further, Dr. Ramanathan and Dr. Collins say, appears to be about 90 degrees Fahrenheit on a monthly average. The ceiling might be pushed upward by two or three degrees, Dr. Ramanathan said, by an unexpectedly large buildup of atmospheric carbon dioxide concentrations. But for that to happen, he said, the concentrations would have to increase tenfold rather than merely double.

The thermostatic process also appears to take place in the tropical Atlantic and Indian Oceans, the scientists say, although the El Nino experiment is in some ways considered the best test of the thermostat hypothesis.

"It's a very simple effect, but no one had demonstrated it effectively until now," said Dr. Andrew J. Heymsfield, an atmospheric physicist at the National Center for Atmospheric Research in Boulder, Colo., who wrote a commentary for Nature on the new research. In an interview, he called the study "first rate" and said he thought its conclusions were correct for the tropical Pacific.

He also called the results "provocative" in suggesting the possibility of a climatic thermostat that might affect human-induced global warming.

But, he said, "it's not at all straightforward." It is not clear, for instance, whether heat produced by global warming at the surface of the continents could be transported to the tropical oceans, where the thermostatic effect might offset it. The tropical oceans, with their enormous amounts of heat energy, drive the planet's climate, and in doing so they create huge, circulating wind currents that transport heat from one part of the earth to another.

Another uncertainty, Dr. Heymsfield said, has to do with the source of the warming in the Ramanathan-Collins study and the source of the human-induced global warming that scientists predict. In the former, the source is sunlight and surface sea currents. In the latter, it is heat-trapping atmospheric gases.

Dr. Ramanathan and Dr. Collins add their own caveats. They say their findings constitute experimental evidence of an important feedback arrangement in the climate system. But there are many others about which little is known, and Dr. Ramanathan says that any number of climatic factors could upset the workings of the mechanism on a global scale.

The findings do suggest, he said, that greenhouse warming, if it is large enough, could cause the much cooler oceans outside the tropics to warm to the maximum, establishing a more or less uniform oceanic temperature throughout the world. This in itself would have profound effects on the world's climate, he said, since it is the differences in temperature among the oceans that largely sets in motion the planetary winds that create and shape regional climate. With these differences eliminated, regional climates could be expected to change profoundly and disruptively. This may be the most significant aspect of the Ramanathan-Collins paper, said Dr. William A. Nierenberg, a geophysicist who is director emeritus of the Scripps Institution and who has expressed doubt that global warming will be as severe as some scientists have predicted. A number of feedbacks might act to limit global warming, he said. "In fact," he said, "the average global temperature change could be almost zero" because of the many feedback mechanisms. But the operation of the mechanisms could nevertheless impose a "severe" price in terms of the regional climate changes they would cause.

Some scientists say the Ramanathan-Collins findings could lead to better predictions about the extent of the expected global warming. The predictions rest largely on computerized mathematical models of the climate system that all scientists agree are crude and flawed. One of the major flaws has to do with the effect of clouds.

Already, the findings cast doubt on some features of the models. For instance, Dr. Ramanathan said, some models predict that if atmospheric carbon dioxide concentrations are doubled, the surface temperature of the western Pacific will rise to about 93 degrees, well above the ceiling that he and Dr. Collins believe they have discerned. "If our hypothesis is valid," Dr. Ramanathan said, "that's not possible."

Diagram: "A Natural Thermostat?" Scientists believe they have identified a climatic thermostat in the tropical Pacific, where clouds act to shut off sunlight and stop the sea surface from warming. They say the thermostat could prevent warming caused by heat-trapping atmospheric gases form spiraling out of control: Diagram 1: As El Nino warmed the ocean surface, water vapor increased substantially, helping trap more heat and increasing the warming Diagram 2: Ten miles high, water vapor condenses into "anvil" cirrus clouds of icy crystals that together cover up to four million miles of the earth's surface, shading the surface and halting to warming. (Source: V. Ramanathan)